Refrigeration cycle apparatus

ABSTRACT

A refrigeration cycle apparatus ( 100 ) includes: a first compressor ( 101 ); a second compressor ( 102 ) connected in parallel with the first compressor ( 101 ) in a refrigerant circuit ( 200 ); a radiator ( 103 ) for cooling a refrigerant compressed by the first and second compressors; an expander ( 104 ) coupled to a rotation shaft of the first compressor ( 101 ); an evaporator ( 105 ) for evaporating the refrigerant expanded by the expander ( 104 ); and a controller ( 115 ). The controller ( 115 ) includes an efficiency enhancing means for performing a first process including a step of changing a rotation speed of the first compressor ( 101 ), as a process for increasing a coefficient of performance (COP) of the refrigeration cycle apparatus ( 100 ), when a temperature of a heat carrier flowing into the radiator ( 103 ) is in a predetermined temperature range, and for performing a second process including a step of changing a rotation speed of the second compressor ( 102 ), as a process for increasing the coefficient of performance, when the temperature of the heat carrier flowing into the radiator ( 103 ) is not in the predetermined temperature range.

TECHNICAL FIELD

The present invention relates to a refrigeration cycle apparatus.

BACKGROUND ART

As described, for example, in JP 2001-116371 A, there has been known arefrigeration cycle apparatus in which an expander and a compressor arecoupled together by a rotation shaft so that the power obtained by theexpander is utilized for driving the compressor to improve thecoefficient of performance (COP). This refrigeration cycle apparatus hasa drawback of being less efficient under operating conditions other thanthe ideal conditions in design. This is because the expander and thecompressor are coupled together by the rotation shaft and therefore theratio between the displacement of the expander and that of thecompressor cannot be changed.

There have been proposals to provide a bypass circuit for bypassing theexpander and to provide a pre-expansion valve on the upstream side ofthe expander in order to obtain a high COP under any operatingconditions. Specifically, when the displacement of the expander isinsufficient, part of the cooled refrigerant is caused to flow throughthe bypass circuit to ensure the required circulation amount of therefrigerant. On the other hand, when the displacement of the expander isexcessive, the refrigerant is decompressed through the pre-expansionvalve to increase the specific volume of the refrigerant previously.

The use of the bypass circuit and the pre-expansion valve, however,reduces the power that can be recovered by the expander, which alsoreduces the COP enhancing effect. To solve this problem, JP 2004-212006A proposes a refrigeration cycle apparatus as shown in FIG. 14. Thisrefrigeration cycle apparatus includes a first compressor 21, anexpander 23 coupled to the first compressor 21, and a second compressor22 placed in parallel with the first compressor 21. When the highpressure of the cycle is higher than a target value, the total of thedisplacement of the first compressor 21 and that of the secondcompressor 22 is larger than the ideal value. Therefore, the rotationspeed of the second compressor 22 is decreased to decrease thedisplacement thereof. Then, the amount of the refrigerant flowingthrough the expander 23 decreases, which brings the high pressure of thecycle closer to the target value. On the other hand, when the highpressure of the cycle is lower than the target value, the rotation speedof the second compressor 22 is increased. Such an adjustment of therotation speed of the second compressor 22 leads to efficient operation.

DISCLOSURE OF THE INVENTION

It may be possible to adjust the rotation speed of the first compressor21 and that of the second compressor 22 alternately to control theoperation, but such a control may raise concerns about the stability ofthe system, and may require more complexity. In view of this, it makessense to control the operation by adjusting the rotation speed of thesecond compressor 22 while fixing the rotation speed of the firstcompressor 21.

As a result of intensive studies, however, the present inventors havefound out that there is another approach to the control for moreefficient operation rather than the increase and decrease of only therotation speed of the second compressor 22.

Specifically, the present invention provides a refrigeration cycleapparatus including:

a first compressor;

a second compressor connected in parallel with the first compressor in arefrigerant circuit;

a radiator for cooling a refrigerant compressed by the first and secondcompressors;

an expander coupled to a rotation shaft of the first compressor;

an evaporator for evaporating the refrigerant expanded by the expander;and

a controller including an efficiency enhancing means for performing afirst process including a step of changing a rotation speed of the firstcompressor, as a process for increasing a coefficient of performance ofthe refrigeration cycle apparatus, when a temperature of a heat carrierto be heated in the radiator is in a predetermined temperature range,and for performing a second process including a step of changing arotation speed of the second compressor, as a process for increasing thecoefficient of performance, when the temperature of the heat carrier tobe heated in the radiator is not in the predetermined temperature range.

According to the present invention, it is judged whether or not thetemperature of the heat carrier to be heated in the radiator is in thepredetermined temperature range, and based on this judgment, it isdetermined which process should be performed, the first process or thesecond process. When the temperature of the heat carrier is in thepredetermined temperature range, the first process is performed. Whenthe temperature of the heat carrier is not in the predeterminedtemperature range, the second process is performed. This technique makesthe operation more efficient rather than mere increase and decrease ofthe rotation speed of the second compressor. The reasons for this effectwill be described later in detail. In addition, this techniqueeliminates the need to adjust the rotation speed of the first compressorand that of the second compressor alternately, which facilitates thecontrol of the respective compressors and also enhances the stability ofthe system.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a configuration of a refrigerationcycle apparatus according to a first embodiment of the presentinvention.

FIG. 2 is a graph showing a relationship between an incoming watertemperature and a COP (at an outside air temperature of 2° C.).

FIG. 3 is a graph showing a relationship between an incoming watertemperature and a COP (at an outside air temperature of 16° C.).

FIG. 4 is a correlation diagram between an outside air temperature andan optimum density ratio (at an incoming water temperature of 35° C.).

FIG. 5 is a correlation diagram between an incoming water temperatureand an optimum density ratio (at an incoming water temperature of 2°C.).

FIG. 6 is a correlation diagram between a high pressure of asupercritical cycle and a COP.

FIG. 7 is a correlation diagram between a high pressure and rotationspeeds of respective compressors in the refrigeration cycle apparatus ofthe first embodiment.

FIG. 8 is a correlation diagram between a high pressure and a rotationspeed of a first compressor in a refrigeration cycle apparatus in whicha second compressor is not provided.

FIG. 9 is a flow chart of a control according to the first embodiment.

FIG. 10A is a graph schematically showing a profile of a COP achieved bythe control shown in the flow chart of FIG. 9.

FIG. 10B is a partially enlarged view of FIG. 10A.

FIG. 11 is a block diagram showing a configuration of a refrigerationcycle apparatus according to a second embodiment of the presentinvention.

FIG. 12 is a graph showing a relationship between an incoming watertemperature and a COP (at an outside air temperature of 2° C.).

FIG. 13 is a flow chart of a control according to the second embodiment.

FIG. 14 is a block diagram showing a configuration of a conventionalrefrigeration cycle apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

As shown in FIG. 1, a refrigeration cycle apparatus 100 of the presentembodiment includes a first compressor 101, a second compressor 102, aradiator 103, an expander 104, and an evaporator 105. These componentsare connected by pipes to form a refrigerant circuit 200. The firstcompressor 101 and the second compressor 102 respectively compress arefrigerant. The radiator 103 cools the refrigerant compressed by thefirst compressor 101 and the refrigerant compressed by the secondcompressor 102. The expander 104 expands the refrigerant cooled by theradiator 103. The evaporator 105 heats the refrigerant expanded by theexpander 104. In the refrigerant circuit 200, the second compressor 102is provided in parallel with the first compressor 101. The refrigerantcircuit branches on the downstream side of the evaporator 105 so thatthe refrigerant is directed to the first compressor 101 and the secondcompressor 102 respectively. The branches of the refrigerant circuit 200are merged with each other on the upstream side of the radiator 103 sothat the total amount of the compressed refrigerant flows into theradiator 103. The refrigerant circuit 200 is filled with the refrigerantsuch as carbon dioxide and hydrofluorocarbon.

When the refrigeration cycle apparatus 100 is applied to a water heater,a water-type heat exchanger can be used for the radiator 103 and anair-type heat exchanger can be used for the evaporator 105. When therefrigeration cycle apparatus 100 is applied to an air conditioner,air-type heat exchangers can be used for both of the radiator 103 andthe evaporator 105. The present embodiment shows an example in which theradiator 103 is a water-type heat exchanger. The radiator 103 has awater circuit 129 therein, and water (heat carrier) flowing through thewater circuit 129 and the refrigerant flowing through the refrigerantcircuit 200 exchange heat in the radiator 103.

The first compressor 101, the second compressor 102, and the expander104 each are a positive displacement fluid machine of scroll type,rotary type, reciprocating type, or the like. The energy released fromthe refrigerant during its expansion is recovered by the expander 104 inthe form of power. The first compressor 101 and the expander 104 arecoupled together by a rotation shaft 123 so that the first compressor101 can use the power recovered by the expander 104.

A motor 110 for driving the rotation shaft 123 is disposed between thefirst compressor 101 and the expander 104. In the present embodiment,the rotation speed of the first compressor 101 always is equal to thatof the expander 104 because they are coupled together by the rotationshaft 123. On the other hand, a dedicated motor 111 is connected to thesecond compressor 102. The rotation speed of the motor 110 can becontrolled separately from that of the motor 111. In other words, therotation speed of the first compressor 101 can be controlledindependently from that of the second compressor 102. Thereby, theconstraint of constant density ratio can be avoided.

The first compressor 101, the motor 110, the rotation shaft 123, and theexpander 104 are accommodated in a common closed casing (not shown).This type of fluid machine is disclosed in WO 2006/035934, for example.Likewise, the second compressor 102 and the motor 111 also areaccommodated in the common closed casing (not shown).

The refrigeration cycle apparatus 100 further includes a controller 115as a means for controlling the operation, a first inverter 125 forsupplying electric power to the motor 110, and a second inverter 127 forsupplying electric power to the motor 111. Specifically, a DSP (DigitalSignal Processor) including an A/D conversion circuit, an input/outputcircuit, an arithmetic and logic unit, a memory, etc. is used as thecontroller 115. The controller 115 controls the inverters 125 and 127 toadjust the rotation speeds of the motors 110 and 111, more specifically,the rotation speeds of the first compressor 101 and the secondcompressor 102.

The refrigerant circuit 200 is provided with a radiator-outlettemperature sensor 112 for detecting the temperature of the refrigerantat the outlet of the radiator 103, and a pressure sensor 117 fordetecting the pressure of the refrigerant at the outlet of the radiator103. An ambient temperature sensor 113 for detecting the ambienttemperature (for example, the outside air temperature) of the evaporator105 is provided in the vicinity of the evaporator 105. The water circuit129 is provided with an incoming water temperature sensor 114 fordetecting the temperature of water (heat carrier) to be heated in theradiator 103. Temperature detecting elements, such as a thermistor and athermocouple, can be used for these temperature sensors. Examples of thepressure sensor 117 include sensors using semiconductor devices. Signalsfrom each of these sensors are inputted to the controller 115.

The suction volume of the first compressor 101 may be different fromthat of the second compressor 102, but it is advantageous when they areequal. This is because the use of the same fluid machines for both thefirst compressor 101 and the second compressor 102 reduces the cost. The“suction volume” means the trapped volume at the time when the suctionstroke is completed.

Next, the operation of the refrigeration cycle apparatus 100 will bedescribed.

First, preliminary experiments are described. The preliminaryexperiments were carried out to examine how the COP of the refrigerationcycle apparatus 100 changed when the refrigeration cycle apparatus 100was applied to a hot water-type heater. In these preliminaryexperiments, the following controls were attempted: (i) a control foradjusting the rotation speed of the first compressor 101 to obtain theoptimum high pressure while fixing the rotation speed of the secondcompressor 102 at a predetermined value; and (ii) a control foradjusting the rotation speed of the second compressor 102 to obtain theoptimum density ratio while fixing the rotation speed of the firstcompressor 101 at a predetermined value. The “optimum high pressure”means the high pressure of the refrigeration cycle at which the COP isexpected to be maximum in terms of the physical properties of therefrigerant. The “high pressure of the refrigeration cycle” means thepressure of the refrigerant discharged from the first compressor 101 andthe second compressor 102 and directed to the expander 104 by way of theradiator 103. The “optimum density ratio” means the ratio Q (Q=ρe/ρc)between the density ρe of the refrigerant at the inlet of the firstcompressor 101 and the density ρc of the refrigerant at the inlet of theexpander 104. The preliminary experiments were carried out under thefollowing conditions. FIG. 2 and FIG. 3 show the results of theexperiments. In each of FIG. 2 and FIG. 3, the vertical axis indicatesthe COP of the refrigeration cycle apparatus 100, and the horizontalaxis indicates the temperature detected by the incoming watertemperature sensor 114, respectively.

Suction volume of the first compressor: 4 cc

Suction volume of the second compressor: 4 cc

Suction volume of the expander: 0.8 cc

Fixed rotation speed of the first compressor: 60 Hz

Fixed rotation speed of the second compressor: 60 Hz

Refrigerant: carbon dioxide

Outside air temperature: 2° C. or 16° C.

In the graphs of FIG. 2 and FIG. 3, monotonically decreasing curves ofthe COP show the results when the rotation speed of the secondcompressor 102 was adjusted to obtain the optimum density ratio whilethe rotation speed of the first compressor 101 was fixed at 60 Hz. Thelower the incoming water temperature is, the higher the rotation speedof the second compressor 102 is. The higher the incoming watertemperature is, the lower the rotation speed of the second compressor102 is. However, the incoming water temperature is not proportional tothe rotation speed of the second compressor 102.

In the graphs of FIG. 2 and FIG. 3, curves having extreme values showthe results when the rotation speed of the first compressor 101 wasadjusted to obtain the optimum high pressure while the rotation speed ofthe second compressor 102 was fixed at 60 Hz. The lower the incomingwater temperature is, the higher the rotation speed of the firstcompressor 101 is. The higher the incoming water temperature is, thelower the rotation speed of the first compressor 102 is. However, theincoming water temperature is not proportional to the rotation speed ofthe first compressor 101. The COP was calculated based on the powerconsumed by the motor, the heated water temperature, the incoming watertemperature, and the amount of hot water supplied.

The results shown in FIG. 2 and FIG. 3 further are reviewed.

Generally, the heat exchange efficiency in the radiator 103 increases asthe incoming water temperature decreases. Therefore, it can be predictedthat the COP tends to increase or decrease monotonically with respect tothe incoming water temperature. Such a tendency can be seen in thecontrol (ii) for adjusting the rotation speed of the second compressor102 to obtain the optimum density ratio while fixing the rotation speedof the first compressor 101 at a predetermined value. However, in thecontrol (i) for adjusting the rotation speed of the first compressor 101to obtain the optimum high pressure while fixing the rotation speed ofthe second compressor 102 at a predetermined value, the COP does notincrease or decrease monotonically and reaches a peak at a specificincoming water temperature. The reasons why these results were obtainedare not obvious, but the coupling of the first compressor 101 and theexpander 104 by the rotation axis 123 and the physical properties ofcarbon dioxide as a supercritical refrigerant seem to affect theseresults.

To enhance the COP of a refrigeration cycle apparatus using, as aworking fluid, a supercritical refrigerant such as carbon dioxide, andincluding a compressor and an expander coupled to the compressor by arotation axis, attention needs to be paid to the following two points.One is to adjust the density ratio Q to the optimum density ratio, andthe other is to adjust the high pressure of the refrigeration cycle tothe optimum high pressure.

If the optimum high pressure is obtained automatically when the actualdensity ratio Q coincides with the optimum density ratio, the best COPshould be obtained only by performing the above-mentioned control (i) or(ii). Reality, however, is different. As is clear from FIG. 2 and FIG.3, the control method for maximizing the COP of the refrigeration cycleapparatus 100 changes depending on the operating conditions. The optimumdensity ratio does not always produce the optimum high pressure.

As shown in FIG. 2, for example, when the outside air temperature is 2°C., if the incoming water temperature is in the range of 35 to 45° C., ahigh COP is obtained by performing the above-mentioned control (i), andif the incoming water temperature is outside that range, a high COP isobtained by performing the above-mentioned control (ii). As shown inFIG. 3, when the outside air temperature is 16° C., if the incomingwater temperature is in the range of 40 to 47° C., a high COP isobtained by performing the control (i), and if the incoming watertemperature is outside that range, a high COP is obtained by performingthe control (ii).

For example, a room heater using the refrigeration cycle apparatus 100has a heating circuit (corresponding to the water circuit 129 shown inFIG. 1) in which hot water circulates. Therefore, the temperature of thewater to be heated in the radiator 103 generally is about 30 to 50° C.Accordingly, it is desirable for the efficient operation of therefrigeration cycle apparatus 100 to switch the control method accordingto the operating conditions such as the incoming water temperature andthe outside air temperature.

The incoming water temperatures T1 and T2, at which the control methodshould be switched, also depend on the fixed rotation speeds of thefirst compressor 101 and the second compressor 102. That is, when thefixed rotation speeds change, the COP profiles also change from theprofiles shown in FIG. 2 and FIG. 3. For example, in the case where theextent to which the capacity of the refrigeration cycle apparatus 100can be controlled is of no importance, it is possible to use the ratedrotation speed of the first compressor 101 as the fixed rotation speedof the first compressor 101 in the control (ii) and to use the ratedrotation speed of the second compressor 102 as the fixed rotation speedof the second compressor 102 in the control (i). On the other hand, inthe case where the incoming water temperatures T1 and T2, at which thecontrol method should be switched, are determined corresponding to theoutside air temperature and the fixed rotation speeds, the capacity ofthe refrigeration cycle apparatus 100 can be controlled to the greaterextent.

Next, the density ratio Q and the optimum density ratio are described indetail.

A relationship represented by the following equation (1) is established:

Vc*Hz1Ve*Hz=Mc*(Hz1/(Hz1+Hz2))*G:Me*G  (1)

where Vc denotes the suction volume of the first compressor 101, Vedenotes the suction volume of the expander 104 , Hz1 denotes therotation speed of the first compressor 101, Hz2 denotes the rotationspeed of the second compressor 102, Mc denotes the specific volume ofthe refrigerant at the inlet of the first compressor 101, Me denotes thespecific volume of the refrigerant at the inlet of the expander 104, andG denotes the circulation amount by weight of the refrigerant in therefrigeration cycle apparatus 100.

When the equation (1) is expanded using the density ρc of therefrigerant at the inlet of the first compressor 101 and the density ρeof the refrigerant at the inlet of the expander 104, the followingequation (2) is obtained:

ρe/ρc=(Vc/Ve)*((Hz1+Hz2)/Hz1)  (2)

Vc/Ve in the right side of this equation is a design value, which isdifficult to be changed arbitrarily. Therefore, in order to obtain adesired density ration Q (Q=ρe/ρc), the rotation speed Hz1 of the firstcompressor 101 and/or the rotation speed Hz2 of the second compressor102 need to be adjusted. Here, as shown in FIG. 4, the optimum densityratio increases as the outside air temperature decreases. As shown inFIG. 5, the optimum density ratio increases as the incoming watertemperature decreases. In addition, the optimum density ratio isaffected by various other factors such as the temperature of therefrigerant drawn into the compressor, the specifications of therefrigeration cycle apparatus, and the amount of the refrigerant filled.

For example, assume that the outside air temperature is 7° C. (wintercondition), and the target value of the density ratio Q (the optimumdensity ratio) is 10. When the suction volume Vc of the first compressor101 is 4 cc, the rotation speed Hz1 of the first compressor 101 is 60Hz, and the suction volume Ve of the expander 104 is 0.8 cc, therotation speed Hz2 of the second compressor 102 is 60 Hz. When theoutside air temperature is 25° C. (summer condition) and the targetvalue of the density ratio Q is 8, the rotation speed Hz2 of the secondcompressor 102 is 36 Hz.

In the present embodiment, the second compressor 102 is controlled toobtain the optimum density ratio. When the rotation speed of the secondcompressor 102 is changed while the rotation speed of the firstcompressor 101 is fixed at a predetermined value, only the flow rate byvolume of the compressed refrigerant can be increased or decreased whilethe flow rate by volume in the expander 104 is maintained constant. Thisis advantageous because the adjustment range of the density ratio isexpanded. This advantage is of particular significance in the placeswhere the day/night difference or seasonal difference in the temperatureis large because the difference in the optimum density ratio also islarge in such places. Furthermore, in the present embodiment, therotation speed of the first compressor 101 is adjusted to obtain theoptimum high pressure. When the rotation speed of the first compressor101 changes, the rotational speed of the expander 104 also changes.Therefore, the use of the first compressor 101 makes it easy to adjustthe high pressure of the refrigeration cycle finely.

Next, the high pressure and the optimum high pressure of therefrigeration cycle are described in detail.

The refrigeration cycle using carbon dioxide as a refrigerant forms asupercritical cycle in which the refrigerant is brought into asupercritical state on the high pressure side (a path from thecompressor to the expander through the radiator). Therefore, as shown inFIG. 6, the COP has a peak with respect to the high pressure. Theoptimum high pressure corresponding to the peak varies depending on thetemperature of the refrigerant at the outlet of the radiator 103, theoutside air temperature, etc. FIG. 6 shows an example in which thedegree of superheat is 5° C. and the incoming water temperature is 35°C.

The optimum high pressure can be calculated based on the state of therefrigeration cycle. Specifically, the optimum high pressure can becalculated based on the detection results of the radiator outlettemperature sensor 112 and the ambient temperature sensor 113. The mosteffective means for changing the high pressure is to change the rotationspeed of the compressor. The high pressure can be adjusted arbitrarilyby changing the rotation speed of the first compressor 101.

Confirmatory experiments were carried out to examine the relationshipsbetween the rotation speeds of the respective compressors and the highpressures at different seasons of the year. FIG. 7 shows the results ofthe experiments. For example, under the intermediate condition, therotation speed of the first compressor 101 was adjusted so that theactual high pressure (data represented by a black rhomb in FIG. 7)coincided with the optimum high pressure (data represented by a cross),and the rotation speed of the second compressor 102 was adjusted toobtain the required heating capacity. As a result of the proper controlof the first compressor 101 and the second compressor 102, the actualhigh pressures were allowed to coincide with the optimum high pressuresthroughout the year including winter, intermediate, and summer seasons.

Furthermore, for the refrigeration cycle apparatus in which the secondcompressor is not provided, the relationship between the rotation speedof the first compressor and the high pressure was examined at differentseasons of the year. FIG. 8 shows the results. The actual high pressurecoincided well with the optimum high pressure under the wintercondition, but under the intermediate and summer conditions, the actualhigh pressure had significantly different values from the optimum highpressure. This is because the first compressor and the expander aredesigned to meet the winter condition. In the case where the secondcompressor is not provided, the density ratio Q always is constant(constraint of constant density ratio), regardless of the rotationspeed. Therefore, the high pressure is left to take its course. A highCOP is obtained only in the winter season, but the COP is poorthroughout the year.

Next, the control procedure for the respective compressors is describedwith reference to the flow chart of FIG. 9. An example in which therefrigeration cycle apparatus 100 is applied to a water heater(including a room heater) is described.

The controller 115 performs the control procedure shown in FIG. 9 atregular intervals. First, in Step 201, the controller 115 judges whetheror not it has received a starting trigger to start up the refrigerationcycle apparatus 100.

The “starting triggers” include a trigger to notify the controller 115that the operation should be started, and a trigger to notify thecontroller 115 that the required capacity of the refrigeration cycleapparatus 100 has been changed. The former trigger occurs, for example,when a user turns the tap on to use hot water, when the room heater isturned on, when the amount of hot water stored in a tank decreases tobelow a predetermined amount, when the hot water storage operation isperformed automatically at midnight, etc. The latter trigger occurs, forexample, when a user changes the preset temperature for room heating,when the degree of heating from “low” to “high, etc. The “requiredcapacity” means the capacity that the refrigeration cycle apparatus 100should provide.

When the refrigeration cycle apparatus 100 is not in operation, or whenits required capacity is changed even during its operation, theinitializing process of Steps 202 to 204 is performed. On the otherhand, when the refrigeration cycle apparatus 100 has already been inoperation and its required capacity is not changed, Steps 202 to 204 areomitted and the processes of Step 205 and the following steps areperformed.

In Step 202, first, the required capacity is calculated based on theuser's instruction or the like that is the information included in thestarting trigger. In the case where the refrigeration cycle apparatus100 is applied to a bath water heater, the “user's instruction” is, forexample, a “reheating operation”, a “hot water adding operation”, or thelike, selected by the user using a remote control. For the “reheatingoperation”, the required capacity is set (for example, at 5 kW) so thathot water of 50° C. is supplied, and for the “hot water addingoperation”, the required capacity is set (for example, at 4 kW) so thathot water of 40° C. is supplied. In the case where the refrigerationcycle apparatus 100 is applied to a room heater, the required capacityis set, for example, according to the room temperature set by the user.In some cases, the controller 115 sets the required capacityautomatically based on parameters such as an outside air temperature, arequired amount of hot water, and an incoming water temperature.

Next, in Step 203, the rotation speeds of the first compressor 101 andthe second compressor 102 are respectively determined so that therequired capacity can be provided. Specifically, the initial rotationspeeds of the first compressor 101 and the second compressor 102 aredetermined previously corresponding to the required capacity. Theinitial rotation speed of the first compressor 101 may be equal to thatof the second compressor 102, or they may be different from each other.The initial rotation speeds may be determined based not only on therequired capacity but also on the detection results of the ambienttemperature sensor 113 and the incoming water temperature sensor 114.Instructions are given to the inverters 125 and 127 so that the firstcompressor 101 and the second compressor 102 operate at the determinedinitial rotation speeds.

Next, in Step 204, the incoming water temperature is detected to set thecontrol mode for the current operation of the refrigeration cycleapparatus 100. As shown in FIG. 10A, when the incoming water temperatureis lower than T1 or higher than T2, the “density ratio control mode” forperforming the control (ii) described above is set and stored in thememory. On the other hand, when the incoming water temperature is in therange of T1 to T2, the “high pressure control mode” for performing thecontrol (i) described above is set and stored in the memory.

Next, in Step 205, in order to recognize the operational state of therefrigeration cycle apparatus 100, signals are obtained from theradiator outlet temperature sensor 112, the ambient temperature sensor113, and the incoming water temperature sensor 114, so that therespective temperatures are detected.

Next, in Step 206, it is judged whether or not the detected incomingwater temperature is in the predetermined temperature range of T1 to T2.The “predetermined temperature range” is a temperature range determinedcorresponding to the ambient temperature of the evaporator 105. Asdescribed with reference to FIG. 2 and FIG. 3, for example, when theoutside temperature is 2° C., the “predetermined temperature range” is35 to 45° C., and when the outside temperature is 16° C., the“predetermined temperature range” is 40 to 47° C.

In the case where the fixed rotation speeds of the respectivecompressors are not constant, the “predetermined temperature range” alsois a temperature range determined corresponding to the fixed rotationspeed of the second compressor 102 when the first process is performedin the high pressure control mode and the fixed rotation speed of thefirst compressor 101 when the second process is performed in the densityratio control mode. As described later, the “first process” is a processincluding a step of changing the rotation speed of the first compressor101. Likewise, the “second process” is a process including a step ofchanging the rotation speed of the second compressor 102.

When the incoming water temperature is in the range of T1 to T2, the COPof the refrigeration cycle apparatus can be maximized by adjusting thehigh pressure of the refrigeration cycle to coincide with the optimumhigh pressure Pm. Therefore, the process proceeds to Step 207, in whichit is judged first whether or not the control mode needs to be switched.When the control mode needs to be switched, the rotation speeds of therespective compressors are adjusted in Step 208.

As shown in FIG. 10A, when the incoming water temperature increasesgradually and exceeds T1, the control mode is switched from the densityratio control mode to the high pressure control mode. That is, as shownin FIG. 10B, the COP profile traces the line from Point P₁ to Point P₂.For example, assume that the rotation speed (fixed rotation speed) ofthe first compressor 101 and the rotation speed of the second compressor102 are 60 Hz and 45 Hz, respectively, at Point P₁. Furthermore, assumethat the rotation speed of the first compressor 101 and the rotationspeed (fixed rotation speed) of the second compressor 102 are 48 Hz and60 Hz, respectively, at Point P₂. Since the compressor to be operated ata fixed rotation speed is changed from the first compressor 101 to thesecond compressor 102, the rotation speeds of the respective compressorsare adjusted in the course of change from Point P₁ to Point P₂. Thecompressor to be operated at a fixed rotation speed can, of course, bechanged from the second compressor 102 to the first compressor 101.Sudden changes in the rotation speeds of the respective compressors maypossibly lead to an unstable cycle. Therefore, it is preferable that therotation speeds of the respective compressors be changed in Step 208 asslowly as possible.

Next, in Step 209, the optimum high pressure (target high pressure) Pm,at which the COP can be maximized, is calculated. As is well known, theoptimum high pressure Pm is related closely to the incoming watertemperature and the radiator outlet temperature. If the correlationequation (or the correlation table) between the incoming watertemperatures and the corresponding optimum high pressures Pm is inputtedpreviously to the controller 115, the optimum high pressure Pm can beobtained based on the detected incoming water temperature. Likewise, theoptimum high pressure Pm can be obtained by using the radiator outlettemperature instead of the incoming water temperature.

Next, in Step 210, the actual high pressure Pd is detected by thepressure sensor 117. The detected high pressure Pd is compared with theoptimum high pressure Pm to determine which is higher. If Pd is lowerthan Pm, the process proceeds to Step 211, in which the rotation speedof the first compressor 101 is increased. If Pd is equal to or higherthan Pm, the process proceeds to Step 212, in which the rotation speedof the first compressor 101 is decreased. The actual high pressure Pdalso can be obtained (estimated) without using the pressure sensor 117.As is well known, the high pressure of the refrigeration cycle isrelated closely to parameters such as the rotation speeds of thecompressors and the outside air temperature. Therefore, a correlationtable, in which the high pressures are described corresponding to suchparameters, may be used to obtain the actual high pressure Pd.

A correlation table, in which the rotation speeds of the firstcompressor 101 required to obtain the optimum high pressure Pm aredescribed corresponding to the incoming water temperatures, may beinputted previously to the controller 115. With reference to thiscorrelation table, the rotation speed of the first compressor 101 can bedetermined uniquely according to the incoming water temperature.

On the other hand, when the incoming water temperature is not in therange of T1 to T2, the COP of the refrigeration cycle apparatus can bemaximized by adjusting the density ratio Q to coincide with the optimumdensity ratio Qm. The process proceeds to Step 213, in which it isjudged first whether or not the control mode needs to be switched. Whenthe control mode needs to be switched, the rotation speeds of therespective compressors are adjusted in Step 214. The process in Steps213 and 214 is the same as the process in Steps 207 and 208 describedabove.

Next, in Step 215, the optimum density ratio Qm (target density ratio),at which the COP can be maximized, is calculated. As described withreference to FIG. 4 and FIG. 5, the optimum density ratio Qm is relatedclosely to the incoming water temperature and the outside airtemperature. If the correlation equation (or the correlation table)between the incoming water temperatures and the corresponding optimumdensity ratios Qm is inputted previously to the controller 115, theoptimum density ratio Qm can be obtained based on the detected incomingwater temperature. Likewise, the optimum density ratio Qm can beobtained by using the outside air temperature instead of the incomingwater temperature.

In Step 216, the actual density ratio Q and the optimum density ratio Qmare compared with each other to determine which is higher. If Q is lowerthan Qm, the process proceeds to Step 217, in which the rotation speedof the second compressor 102 is increased. If Q is equal to or higherthan Qm, the process proceeds to Step 218, in which the rotation speedof the second compressor 102 is decreased. The actual density ratio Qcan be calculated according to the above-mentioned equation (2).

After the processes in Steps 206 to 218 are performed, the process forcorrecting the rotation speed of the first compressor 101 or therotation speed of the second compressor 102 may be performed.Specifically, it is judged whether or not the rotation speeds of therespective compressors are necessary and sufficient to provide therequired capacity, because the rotation speed of the first compressor101 or the second compressor 102 has changed. If the capacity isinsufficient, the rotation speeds of the respective compressors areincreased by multiplying the rotation speeds thereof by correctioncoefficients enough to compensate for the shortfall in the capacity.Likewise, if the capacity is excessively high, the rotation speeds ofthe respective compressors are decreased by multiplying the rotationspeeds thereof by correction coefficients enough to reduce the excess ofthe capacity. In other words, the controller 115 further includes ameans for correcting the current rotation speed of the first compressor101 and/or the current rotation speed of the second compressor 102,after the first process (Steps 209 to 212) and the second process (Steps215 to 218) are performed, so that the refrigeration cycle apparatus 100can provide the required capacity. This correction allows therefrigeration cycle apparatus 100 to operate with its necessary andsufficient capacity while maintaining the COP at a high level. Since thecorrection changes the fixed rotation speeds, the temperature range ofT1 to T2 also changes.

As described above, the controller 115 includes an efficiency enhancingmeans for performing the first process, as a process for increasing theCOP of the refrigeration cycle apparatus 100, when the incoming watertemperature is in the predetermined temperature range of T1 to T2, andfor performing the second process, as a process for increasing the COP,when the incoming water temperature is not in the predeterminedtemperature range of T1 to T2. The “first process” (Steps 209 to 212) isa process including a step of changing the rotation speed of the firstcompressor 101. The “second process” (Steps 215 to 218) is a processincluding a step of changing the rotation speed of the second compressor102. This switching between the two control methods (control modes)according to the operational state of the refrigeration cycle apparatus100 allows the COP to be maintained at a high level under all theoperating conditions.

Steps 211 and 212 in the first process are steps of changing therotation speed of the first compressor 101 so that the pressure Pd ofthe refrigerant on the high pressure side of the refrigeration cycleapproaches the optimum high pressure Pm at which the COP can bemaximized (optimum high pressure control). Steps 217 and 218 in thesecond process are steps of changing the rotation speed of the secondcompressor 102 so that the density ratio Q between the density ρe of therefrigerant at the inlet of the expander 104 and the density ρc of therefrigerant at the inlet of the first compressor 101 (or the secondcompressor 102) approaches the optimum density ratio Qm at which the COPcan be maximized (optimum density ratio control).

Specifically, in the first process, the high pressure Pd and the optimumhigh pressure Pm are compared to judge whether or not the current highpressure Pd needs to be changed. The rotation speed of the firstcompressor 101 is increased when the high pressure Pd is excessivelylow, and the rotation speed of the first compressor 101 is decreasedwhen the high pressure Pd is excessively high. In the second process,the density ratio Q and the optimum density ratio Qm are compared tojudge whether or not the current density ratio Q needs to be changed.The rotation speed of the second compressor 102 is increased when thedensity ratio Q is excessively small, and the rotation speed of thesecond compressor 102 is decreased when the density ratio Q isexcessively large. The switching between the first process and thesecond process is performed appropriately, the COP can be maintained ata high level under all the operating conditions.

In the present embodiment, the controller 115 further includes aninitializing means for setting initial rotation speeds of the firstcompressor 101 and the second compressor 102 so that the refrigerationcycle apparatus 100 can provide a required capacity, on the conditionthat the controller 115 receives a starting trigger to start up therefrigeration cycle apparatus 100. After the refrigeration cycleapparatus 100 is started up, if the incoming water temperature is in thepredetermined temperature range of T1 to T2, the first process (Steps209 to 212) is performed based on the high pressure Pd in the initialoperation state in which the first compressor 101 and the secondcompressor 102 respectively are operating at the initial rotationspeeds. If the incoming water temperature is not in the predeterminedtemperature range of T1 to T2, the second process (Steps 215 to 218) isperformed based on the density ratio Q in this initial operation state.This allows a smooth starting of the refrigeration cycle apparatus 100.

As shown in FIG. 10A, as a result of switching between the density ratiocontrol mode and the high pressure control mode at the temperatures T1and T2, the COP decreases monotonically with an increase in the incomingwater temperature in the range of incoming water temperatures lower thanthe temperature T1 and in the range higher than the temperature T2. Inthe temperature range of T1 to T2, the COP increases and decreases withan increase in the incoming water temperature to show one extreme value.

In the present embodiment, in Step 210, the high pressure Pd and theoptimum high pressure Pm are compared with each other to determine whichis higher. The optimum high pressure Pm may have a certain range ofvalues. Specifically, the rotation speed of the first compressor 101 isdecreased when the high pressure Pd exceeds the optimum high pressurePm+α, and the rotation speed of the first compressor 101 is increasedwhen the high pressure Pd falls below the optimum high pressure Pm−α.Likewise, the optimum density ratio Qm may have a certain range ofvalues. Specifically, the rotation speed of the second compressor 102 isdecreased when the density ratio Q exceeds the optimum density ratioQm+α, and the rotation speed of the second compressor 102 is increasedwhen the density ratio Q falls below the optimum density ratio Qm−α.These dead zones eliminate the need to change the rotation speedsfrequently even if the high pressure Pd fluctuates slightly. The similardead zones may be provided in the temperatures T1 and T2.

Second Embodiment

As shown in FIG. 11, a refrigeration cycle apparatus 300 of the presentembodiment differs from the first embodiment in that the former furtherincludes an injection circuit 132 having a flow control valve 134. Theinjection circuit 132 connects the outlet of the radiator 103 to theintermediate pressure portion of the expander 104 through the flowcontrol valve 134. A valve controller 136 for adjusting the opening ofthe flow control valve 134 is connected to the controller 115. The flowrate of the refrigerant flowing through the injection circuit 132 variesdepending on the opening of the flow control valve 134. The“intermediate pressure portion” is provided to mix a refrigerant in theprocess of expansion with a high-pressure refrigerant. Typically, theintermediate pressure portion is an opening portion facing the expansionchamber.

As described in the first embodiment, when the outside air temperatureis 25° C. (summer condition), the optimum density ratio Qm is 8, and therotation speed Hz2 of the second compressor 102 is 36 Hz. It is notpreferable, however, from the viewpoint of reliability to operate thesecond compressor 102 at a low rotation speed. It is desirable tooperate the second compressor 102 at a rotation speed (for example 60Hz) at which the efficiency of the motor is maximized. According to thepresent embodiment, in which the excess refrigerant is caused to flowthrough the injection circuit 132 while the second compressor 102 isbeing operated at about the rated rotation speed, the COP can bemaintained at a high level.

As in the first embodiment, the control method (control mode) formaximizing the COP of the refrigeration cycle apparatus 300 is switchedaccording to the operating conditions. As shown in FIG. 12, according tothe refrigeration cycle apparatus 300 of the present embodiment, forexample, when the outside air temperature is 2 C, if the incoming watertemperature is in the range of 36 to 43° C., the control (i) describedin the first embodiment is performed to obtain a higher COP. If theincoming water temperature is outside this range, the control (ii) isperformed to obtain a higher COP. This suggests that the temperaturerange at which the control method should be switched to obtain a higherCOP depends not only on the radiator outlet temperature and the outsideair temperature but also on the configuration of the refrigeration cycleapparatus.

FIG. 13 is a flow chart of the control procedure performed in thepresent embodiment. Steps 301 to 316 are the same as Steps 201 to 216described in the first embodiment. In Step 316, the current densityratio Q and the optimum density ratio Qm are compared with each other,and if Q is lower than Qm, the process proceeds to Step 317, in whichthe rotation speed of the second compressor 102 is increased. If Q isequal to or higher than Qm, the process proceeds to Step 318, in whichthe rotation speed of the second compressor 102 is decreased and theopening of the flow control valve 134 is increased. Even when the actualdensity ratio Q cannot coincide with the optimum density ratio Qm simplyby adjusting the rotation speed of the second compressor 102, the flowcontrol valve 134 provided in the injection circuit 132 serves toincrease the COP of the refrigeration cycle apparatus 300.

(Modification)

At the refrigerant-side outlet of the radiator 103, there is a closerelationship between the temperature of the refrigerant and thetemperature of water. For example, in a water heater, the followingrelationship is established:

(refrigerant temperature at radiator outlet)≈(incoming watertemperature+5° C.).

Therefore, the temperature of the water to be heated in the radiator 103(incoming water temperature) may be detected indirectly based on thedetection result of the radiator outlet temperature sensor 112. In thiscase, since the incoming water temperature sensor 114 can be omitted,which contributes to cost reduction. It also is conceivable to detectthe incoming water temperature indirectly based on the outside airtemperature. Specifically, the temperature of the refrigerant at theoutlet of the radiator 103 or the outside air temperature may be usedinstead of the incoming water temperature to perform a series ofcontrols. In the case where the radiator 103 is a heat exchanger otherthan a water-type heat exchanger (for example, an air-type heatexchanger), the temperature of a heat carrier, such as air, to be heatedin the radiator 103 can be used instead of the incoming watertemperature.

INDUSTRIAL APPLICABILITY

The present invention can be applied to a refrigeration cycle apparatusfor various uses, such as a water heater, a room heater, a bathroomdryer, and an air conditioner.

1. A refrigeration cycle apparatus comprising: a first compressor; asecond compressor connected in parallel with the first compressor in arefrigerant circuit; a radiator for cooling a refrigerant compressed bythe first and second compressors; an expander coupled to a rotationshaft of the first compressor; an evaporator for evaporating therefrigerant expanded by the expander; and a controller including anefficiency enhancing means for performing a first process including astep of changing a rotation speed of the first compressor, as a processfor increasing a coefficient of performance of the refrigeration cycleapparatus, when a temperature of a heat carrier to be heated in theradiator is in a predetermined temperature range, and for performing asecond process including a step of changing a rotation speed of thesecond compressor, as a process for increasing the coefficient ofperformance, when the temperature of the heat carrier to be heated inthe radiator is not in the predetermined temperature range.
 2. Therefrigeration cycle apparatus according to claim 1, wherein thepredetermined temperature range is a temperature range that isdetermined corresponding to an ambient temperature of the evaporator. 3.The refrigeration cycle apparatus according to claim 2, wherein thepredetermined temperature range further is a temperature range that isdetermined corresponding to the rotation speed of the second compressorin the first process and to the rotation speed of the first compressorin the second process.
 4. The refrigeration cycle apparatus according toclaim 1, wherein the step of changing the rotation speed of the firstcompressor is a step of changing the rotation speed of the firstcompressor so that a pressure Pd of the refrigerant on a high pressureside of a refrigeration cycle approaches an optimum high pressure Pm atwhich the coefficient of performance can be maximized, and the step ofchanging the rotation speed of the second compressor is a step ofchanging the rotation speed of the second compressor so that a densityratio Q between a density ρe of the refrigerant at an inlet of theexpander and a density ρc of the refrigerant at an inlet of thecompressor approaches an optimum density ratio Qm at which thecoefficient of performance can be maximized.
 5. The refrigeration cycleapparatus according to claim 4, wherein in the first process, theefficiency enhancing means compares the pressure Pd and the optimum highpressure Pm to judge whether or not the current pressure Pd needs to bechanged, and increases the rotation speed of the first compressor whenthe pressure Pd is excessively low and decreases the rotation speed ofthe first compressor when the pressure Pd is excessively high, and inthe second process, the efficiency enhancing means compares the densityratio Q and the optimum density ratio Qm to judge whether or not thecurrent density ratio Q needs to be changed, and increases the rotationspeed of the second compressor when the density ratio Q is excessivelysmall and decreases the rotation speed of the second compressor when thedensity ratio Q is excessively large.
 6. The refrigeration cycleapparatus according to claim 5, wherein the controller further includesan initializing means for setting initial rotation speeds of the firstcompressor and the second compressor so that the refrigeration cycleapparatus can provide a required capacity, on the condition that thecontroller receives a starting trigger to start up the refrigerationcycle apparatus.
 7. The refrigeration cycle apparatus according to claim1, further comprising a radiator outlet temperature sensor for detectinga temperature of the refrigerant at an outlet of the radiator, whereinthe temperature of the heat carrier to be heated in the radiator isdetected indirectly based on the temperature detected by the radiatoroutlet temperature sensor.
 8. The refrigeration cycle apparatusaccording to claim 1, further comprising an injection circuit includinga flow control valve, the injection circuit connecting an outlet of theradiator to an intermediate pressure portion of the expander through theflow control valve, wherein the second process further includes a stepof changing an opening of the flow control valve.